Cellular localization of alkaline phosphatase activity is being studied at the early mineralization sites adjacent to the dentino-enamel junction of the rat incisor. Histochemical studies of phosphatase activity by light and electron microscopy is related to types of cells and their morphological development, and to the degree of matrix mineralization process. Continuing efforts are being made to improve our methods of tissue preparation and to increase the precision of enzyme localization and tissue element analysis.